In three-layer co-extruded blown-film lines producing polypropylene-based retort pouch structures, the sealant skin is frequently a 10–15 µm layer of metallocene VLDPE coextruded over a homopolymer polypropylene core. COHERE VLDPE 8170D, with datasheet-typical density of 0.870 g/cm³ per ISO 1183-1:2019 and melt flow rate of 0.9 dg/min at 190°C/2.16 kg per ISO 1133-1:2022, provides a seal-initiation temperature below 85°C when tested on a 35 µm sealant web against a polypropylene cast substrate per ASTM F1921. On a 65 mm grooved-feed extruder with 28:1 L/D and a three-layer spiral mandrel die, the layer distribution is set at 15/70/15 by gravimetric hoppers, and the barrel profile from zone 1 to zone 6 is 160/175/190/195/200/205°C. The outer skin should not be processed above 215°C because the narrow low-density chain segments begin to produce gels at extended residence time; field observations on a 65 mm line running at 180 kg/h show that gel counts per 1.0 m² of film increase from 2–3 to 8–10 when melt temperature exceeds 220°C. A die gap of 1.8 mm and blow-up ratio of 2.3:1 maintain frost line height at 420–460 mm, which stabilizes the bubble and limits orientation anisotropy. Haze of a 40 µm three-layer structure is 4–6% per ASTM D1003; dart drop impact on the same structure is 280–350 g per ASTM D1709 Method A. The sealant performance is monitored by hot-tack force and ultimate seal strength: hot-tack at 110°C on a 25 µm sealant layer is 2.0–2.8 N/25 mm per ASTM F1921, while ultimate seal strength after 0.5 s dwell at 120°C reaches 8.5–11.0 N/25 mm per ASTM F88. Antiblock packages should be restricted to synthetic silica at 2,000–4,000 ppm because talc at equal loading raises haze by 2–3 percentage points and disturbs seal clarity. No corona treatment of the sealant skin is permitted above 42 dyn/cm; excessive surface oxidation creates a weak boundary layer that reduces seal strength by 12–18% after 90 days of storage at 25°C and 60% RH.

What Happens When COHERE VLDPE 8170D Replaces Butene-LLDPE in a PP Impact Modification Package?

Addition of COHERE VLDPE 8170D at 10–20 wt% to an injection-grade PP homopolymer with MFR 12 g/10 min at 230°C/2.16 kg produces a phase-separated blend in which the metallocene VLDPE domains are smaller than butene-LLDPE domains at equivalent compounding energy. In a 26 mm co-rotating twin-screw extruder with 40:1 L/D, screw speed 400–450 min⁻¹, barrel temperature 190–205°C, and strand pelleting via an underwater pelletizer with water temperature 18–22°C, the notched Charpy impact strength at 23°C measured per ISO 179-1/1eA increases from 3.0 kJ/m² for the neat homopolymer to 10–14 kJ/m² at 15 wt% loading. At −20°C the same blend retains 5.5–7.5 kJ/m², whereas a butene-LLDPE control at equal loading remains below 4.0 kJ/m² because of coarser dispersed domains. The phase morphology observed on cryofractured surfaces after acid etching shows number-average domain diameters below 1.2 µm at 15 wt%, increasing to 2.5–3.5 µm at 30 wt%, which explains the discontinuous reduction in flexural modulus. The flexural modulus per ISO 178:2019 drops from 1,450 MPa for the neat PP to 1,050 MPa at 15 wt% and to 780 MPa at 30 wt%; tensile stress at yield per ISO 527-2:2012 falls from 33 MPa to 23 MPa over the same range. Processors should limit the VLDPE addition to 25 wt% when the modified PP is intended for under-hood structural brackets, because the blend begins to exhibit pellet agglomeration in railcar unloading after 30 days at ambient temperatures above 35°C, and screw conveying in a 40 mm injection moulding machine with general-purpose screw becomes inconsistent when pellet surface tack exceeds 0.15 N/mm². No anhydride-grafted compatibilizer is required below 20 wt%, but addition of 0.5–1.0 wt% maleated PP is used when the VLDPE content exceeds 25 wt% to reduce delamination in thick-walled parts. The blend has an upper processing temperature boundary of 230°C; residence times above 8 min at 240°C initiate thermal scission in the VLDPE phase, producing unidentified low-molecular-weight fractions that increase plate-out on core pins during injection moulding.

COHERE VLDPE 8170D loading (wt%)	Notched Charpy at 23°C (kJ/m²)	Notched Charpy at −20°C (kJ/m²)	Flexural modulus (MPa)	Tensile yield stress (MPa)
0	3.0	1.8	1,450	33
10	9–12	5–6	1,180	27
20	14–18	7–9	950	24
30	18–22	9–11	780	21

Measurements follow ISO 179-1/1eA for notched Charpy impact, ISO 178:2019 for flexural modulus, and ISO 527-2:2012 for tensile yield stress.

Extrusion Coating and Lamination onto Porous and Flexible Substrates

A 45 mm single-screw extruder with 30:1 L/D, barrier feed section, and mixing pins is used to apply COHERE VLDPE 8170D at 20–30 µm coating weight onto corona-treated BOPET, aluminium foil, and kraft paper. Melt temperature at the die is held between 230°C and 245°C, while the die width is 900 mm and the air gap is set at 180–220 mm. At line speed 180–250 m/min, neck-in measured from die width to coating width is 28–35 mm per side; at 300 m/min, neck-in increases to 42–48 mm because the strain-hardening capacity of the metallocene VLDPE is lower than that of LDPE extrusion-coating grades. The melt curtain remains stable when the draw-down ratio is kept between 8:1 and 12:1; at draw-down ratios above 15:1, edge weave and melt resonance appear as parallel thickness bands with spacing of 30–50 mm. Adhesion to BOPET after corona treatment at 45 dyn/cm is 3.5–5.0 N/15 mm when sealed at 210°C and 0.3 s dwell per ASTM F904. For aluminium foil, a primer layer of ethylene-acrylic acid copolymer and a foil preheating at 120°C raise adhesion above 8 N/15 mm; without primer the adhesion falls below 2.5 N/15 mm because the VLDPE lacks carboxylic functionality to bond with the oxide surface. The low melting point of the resin allows a reduction in lamination head temperature by 10–15°C relative to LDPE at equal coating weight, which limits the thermal load on heat-sensitive polyamide films and reduces odour in retortable lidding film. The processing boundary is defined by gel formation: a melt temperature of 255°C at residence time 6 min produces visible gel clusters above 300 µm in the coating, corresponding to a 4–5% loss in seal strength. On production lines, the screw speed should not exceed 90 min⁻¹ for a 45 mm extruder processing at 230°C, because viscous heat dissipation pushes melt temperature above 250°C and triggers local crosslinking in high-shear zones near the screw tip. The extruder barrel profile is 140/180/210/230/240/240°C, and the adapter and die are maintained at 240°C.

Formulation of hot-melt pressure-sensitive adhesives for cold-chain label facestock uses COHERE VLDPE 8170D as a base polymer at 25–40 wt% in combination with a C5/C9 hydrogenated tackifier, a paraffinic process oil, and 1.0–1.5 wt% phenolic antioxidant. The resin is first masticated in a 5 L sigma-blade mixer at 160°C under nitrogen blanket for 20 min, then the tackifier is added in three equal portions at 15 min intervals to prevent phase inversion. Viscosity is measured by a Brookfield thermosel viscometer with SC4-27 spindle at 180°C per ASTM D3236-15; a formulation containing 35 wt% VLDPE, 45 wt% tackifier, and 20 wt% oil gives a viscosity of 4,000–6,000 mPa·s, while increasing tackifier to 50 wt% lowers viscosity to 2,800–3,800 mPa·s. Ring-and-ball softening point per ASTM E28-99 is between 85°C and 98°C for the 35/45/20 formulation and drops to 72–80°C when oil content exceeds 25 wt%. Loop tack on stainless steel at 23°C per ASTM D6195-03 reaches 10–14 N/25 mm, but falls below 6 N/25 mm when tackifier content drops below 35 wt% because the VLDPE segment chain mobility is insufficient to wet the substrate. Static shear adhesion at 40°C with 1 kg load is 8–12 h for the same formulation; below 20 wt% oil the shear time exceeds 24 h but the low-temperature application window narrows. The adhesive is applied at 160–175°C through a slot-die coater at 15–25 g/m² onto silicone-coated release liner and then laminated to BOPP facestock. Continuous coaters running above 190°C show progressive colour development from water-white to Gardner 3–4, which is attributable to tackifier degradation in the presence of residual unsaturation; therefore nitrogen blanketing of the feed hopper is required above 170°C. The operational boundary is the incompatibility with high-vinyl acetate EVA hot-melt additives above 10 wt%, which produces a two-phase melt with visible surface mottling and reduces peel adhesion by 15–20%. For cold-chain label applications down to −25°C, the VLDPE content should remain above 30 wt%, because lower contents produce adhesive face-stock delamination at freezer temperatures.

When Chemical Blowing Agents Are Used at Low Melt Temperatures for Resilient Foam Sheet

Activation of azodicarbonamide at 160–205°C overlaps the melt-processing window of COHERE VLDPE 8170D, enabling direct extrusion of closed-cell resilient foam sheet without precompounding the blowing agent. A 50 mm single-screw extruder with 30:1 L/D, oil-cooled screw, and a coat-hanger sheet die of 600 mm width is run at 160–175°C in the metering zone, with the die temperature held at 155–160°C. The blowing agent is dry-tumbled at 0.6–1.2 wt% on a low-intensity ribbon blender for 10 min before feeding. The density of the extruded foam measured per ASTM D792 drops from 0.870 g/cm³ to 0.48–0.55 g/cm³ at 1.0 wt% azodicarbonamide; at 1.2 wt% the density falls to 0.38–0.42 g/cm³ but cell coalescence becomes visible as surface pits because the melt strength of the metallocene VLDPE is insufficient to contain gas expansion at the die exit. The tensile strength of the foam measured per ASTM D3575 is 2.2–2.8 MPa at 0.55 g/cm³, and tear resistance per ASTM D3575 is 35–45 kN/m. Compression set at 25% deflection and 23°C after 24 h recovery is 18–24% per ISO 815-1:2019, indicating suitability only for non-structural cushioning. The processing boundary is narrow: die temperature below 150°C causes premature gas decomposition in the die and sharkskin, while die temperature above 170°C results in gas loss through the melt curtain and a density increase to 0.70–0.78 g/cm³. Published data for this specific configuration is limited, but industrial trials on a 50 mm line indicate that 0.8 wt% blowing agent is the maximum safe loading for continuous operation above 48 h without die lip build-up. The resin should not be processed with endothermic foaming agents requiring temperatures above 200°C, because the upper processing limit of the VLDPE lies below the decomposition plateau of those agents.

In high-clarity PE film masterbatch production, COHERE VLDPE 8170D is used as a carrier for erucamide slip and synthetic silica antiblock at total additive loadings of 15–25 wt%. The carrier is selected because its 0.870 g/cm³ density and 0.9 dg/min MFR at 190°C/2.16 kg permit dispersion at melt temperatures 160–190°C, which is low enough to prevent thermal decomposition of erucamide at 220°C. A 25 mm co-rotating twin-screw extruder with 36:1 L/D, screw speed 300–350 min⁻¹, and a filter pack of 150/250/150 mesh is used to produce strand pelletized masterbatch. The pressure before the screen pack is 60–80 bar; a pressure rise above 110 bar indicates filter blinding by undispersed silica agglomerates, which correlates with surface roughness in the final film. The slip migration behaviour of erucamide from the VLDPE carrier into LLDPE film is measured by coefficient of friction after 24 h aging at 23°C per ASTM D1894; a film containing 5 wt% of a 20% erucamide masterbatch reaches a kinetic coefficient of friction of 0.15–0.20, while a butene-LLDPE carrier at equal loading remains at 0.28–0.35 under the same aging time. The carrier also reduces torque in the masterbatch extruder by 10–15% compared with a 0.918 g/cm³ LDPE carrier. Zinc stearate should be avoided above 1.5 wt% in the formulation because it forms insoluble zinc erucamide complexes that bloom as white particulate on the film surface after 72 h at 40°C. The operational boundary is residence time above 6 min at melt temperatures above 200°C, which degrades the antioxidant package and reduces the oxidative induction time of the carrier to below 20 min at 210°C per ISO 11357-6:2018.

High-Clarity Stretch Film Layers for Cold Storage Wrapping

A 55 mm blown-film line with dual-lip air ring and internal bubble cooling is used to produce 15–25 µm stretch film containing COHERE VLDPE 8170D in the core layer at 30–50 wt% with a linear low-density polyethylene skin. The frost line is maintained at 500–550 mm above the die, bubble blow-up ratio is 2.5:1, and melt temperature at the die is 185–195°C. The VLDPE reduces puncture propagation in the machine direction: puncture force per ASTM D5748 on 20 µm film is 18–24 N at 40 wt% core layer, compared with 12–15 N for an equivalent LLDPE film. The load retention at 200% stretch on an A-frame pallet wrapper is 55–65% after 1 h at 23°C; at −5°C the same film retains 50–58%, whereas a conventional butene-LLDPE film drops to 35–40%. The film haze measured per ASTM D1003 remains at 1.8–2.5%, and the dart drop impact per ASTM D1709 Method A is 110–140 g for 20 µm. The outer skin must contain an antiblock level of 3,000–6,000 ppm to prevent blocking during roll storage; however, slip migration from the core layer in chilled conditions below 5°C is retarded, and the static coefficient of friction per ASTM D1894 after 24 h at 5°C rises to 0.40–0.55, requiring stretch-film unwinding equipment with active tension control. The upper processing boundary is bubble instability above 220°C; the lower boundary is 175°C, below which the melt pressure at the die becomes erratic. The resin should not be used as a skin layer in direct contact with high-speed rollers above 400 m/min, because its low density creates a tacky surface that accumulates dust and increases roll drag.